Method for the synthesis of a zero-valent metal micro- and nanoparticles in the presence of a noble metal

Abstract

The invention regards a method for the synthesis of Zero-Valent metal micro- and nanoparticles, in which a first aqueous solution (SOL1) of a salt of a noble metal (A) is mixed with a third neutral or basic aqueous solution (SOL3) of an inorganic sulphur-based reducing agent (C), and wherein the mixture thus obtained is added to a second aqueous solution (SOL2) of a salt of a transition metal (B) and a second aliquot of the inorganic reducing agent; such method provides that the amount of the inorganic reducing agent (C) is in a stoichiometric excess in the reduction reaction to Zero-Valent of both the salt of the noble metal (A) contained in the first solution (SOL1) and the salt of the transition metal (B) contained in the second solution (SOL2). The invention also regards Zero-Valent micro and nanoparticles, preferably bimetallic, obtained with the above method. More generally, the invention regards a method for reduction of a transition metal (B) to Zero-Valent metal by an inorganic reducing agent (C), by prior or concurrent reduction of a noble metal (A), wherein the amount of inorganic reducing agent (C) is in stoichiometric excess in the reduction reaction to Zero-Valent of both the noble metal (A) and the transition metal (B). The present invention finds preferred and advantageous application in the remediation and / or the treatment of contaminated water containing at least one polluting substance. The preferred embodiment of the present invention provides that the noble metal (A) is silver, that the transition metal (B) is iron and / or manganese, and the inorganic reducing agent (C) is chosen from borohydrides, dithionites and bisulphites.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This application is the 35 U.S.C. § 371 national stage application of PCT Application No. PCT / IB2018 / 059337, filed Nov. 27, 2018, where the PCT claims priority to, and the benefit of, Italian application no. 102017000136603, filed Nov. 27, 2017, both of which are herein incorporated by reference in their entireties.TECHNICAL FIELD[0002]The present invention relates generally to the field of anti-pollution and more specifically to the field of remediation of contaminated aquifers and the treatment of wastewater.[0003]The present invention has the object of providing a method for the synthesis of Zero-Valent metal micro- and nanoparticles, and also the Zero-Valent micro and nanoparticles, preferably bimetallic, obtained with such method.[0004]More generally, the present invention has the object of providing a method for the reduction of a transition metal to a Zero-Valent metal.[0005]Other areas of application of the present invention, in a...

AI Technical Summary

Benefits of technology

[0040]simplifying, optimizing and improving the production process for the chemical reduction of Zero-Valent metal-based nanomaterials, reducing the operational complexity and increasing the environmental compatibility thereof;

[0061]technical and economic advantage, due to the presence of the noble metal that promotes the reduction of the transition metal, linked to the possibility of performing the synthesis process in very simple operating conditions, without the need of degassing the solutions and generating a controlled atmosphere into the reactor.

Problems solved by technology

Current methods for the production of micro- and nanoparticles of Zero-Valent metals, and in particular of microscopic and nanoscopic Zero-Valent iron, are either thermal reduction processes for the massive production of reactant material, or chemical reduction ones, limited to the laboratory-scale applications.

); such processes are highly energy-intensive, and the produced material has a high cost.

The limits and disadvantages of such processes are linked to the complexity of synthesis conditions and of the equipment required for a correct execution of the process.

In particular, with regard to the examples described in the patent documents, namely in U.S. Pat. No. 2,426,148 and in U.S. Pat. No. 6,974,493, they exhibit a high complexity of the process and of the equipment required for the development of the reduction, as well as a high energy demand, which leads to high operating costs.

The above processes known from the scientific literature, while allowing to obtain materials with suitable characteristics, have disadvantages in terms of industrial-scale applications, due to the difficulties of both realization and operation of such systems; furthermore, in order to obtain micro- and nanoparticles according to these processes, the material grinding should be adopted.

More particularly, the chemical reduction of Zero-Valent iron is generally realised by sodium borohydride, NaBH4; such process leads to the rapid production of Zero-Valent metals, but generates large quantities of gaseous hydrogen (highly flammable, at risk of explosion) and waste characterized by the presence of toxic and / or carcinogenic by-products (e.g., borates); furthermore, this process turns out to be economically unattractive because of the high cost of the reducing agent used, NaBH4, and the cost of treatment and disposal of the waste produced during the synthesis step.

As an alternative to sodium borohydride, NaBH4, the use of reducing compounds with a lower environmental impact has been proposed, such as dithionites or plant extracts; these materials have a significantly lower cost than NaBH4, with a consequent notable reduction in the unit cost of the Zero-Valent iron produced, but the use of these reagent materials requires that the reduction takes place in sealed reactors, in the presence of deaerated water and modified atmosphere with a low oxygen content, an aspect which strongly limits the practicality and scalability for large-scale production.

Sometimes, Zero-Valent iron particles produced by the above-mentioned processes are doped with noble metals in trace amounts, mostly platinum, in order to increase their effectiveness of contaminant removal and to reduce considerably the total time, since the noble metal acts as a catalyst for the reduction and dehalogenation reactions of many challenging polluting compounds; the functionalization with precious metals, which is usually performed downstream the Zero-Valent iron synthesis, however, generates an increase in the unit cost of the Zero-Valent iron.

As discussed above, the main limits and the disadvantages of the above-mentioned processes reside mainly in high production costs, the need to use degassed water and inert atmosphere, the production of toxic compounds and / or at high risk for human health.

In particular, patent documents US2003121364 and WO02062509 perform at high temperature using organic solvents; on the other hand, the document EP2125271 uses a “liquid crystal” step that requires strict conditions, which complicate the synthesis process.

However, this process requires to implement the particle synthesis in the presence of a controlled atmosphere, depleted in oxygen.

This makes the synthesis procedure technologically complex due to the need of using pressurized reactors.

Therefore, the quantity and quality of the Zero-Valent metal produced are not always satisfactory and compatible with the specific application.

In addition, the additional organic substances naturally present in fruit extracts can be adsorbed on the metallic particle surface, limiting the reactivity against contaminant substances.

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